Direct conversion system has been attracting a keen attention as the receiving system for use in a portable RF receiving apparatus. Japanese Patent Unexamined Publication No. 2003-134183 discloses a direct conversion system receiving apparatus suitable to low-voltage power supply.
A conventional portable television receiver which employs the above direct conversion receiving system is described in the following referring to the drawing. FIG. 4 is a block diagram of RF receiving apparatus 201 used in a conventional portable television receiver. As shown in FIG. 4, conventional RF receiving apparatus 201 includes antenna 203, electronic tuner 205 for tuning in a desired channel among the signals received by antenna 203, demodulation unit 207 for demodulating I and Q signals outputted from electronic tuner 205, decoder 209 for decoding those signals demodulated at demodulation unit 207, and display 211 and speaker 213 to which the outputs of decoder 209 are supplied.
RF signals incoming to antenna 203 are led to input terminal 215 of portable television receiver 201's electronic tuner 205. Bandpass filter 217 allows only receiving band frequency to pass through, among those signals inputted to input terminal 215. Those passed through bandpass filter 217 are amplified by amplifier 219 to a certain specific signal level, and delivered to mixing circuit 221 of direct conversion system.
Mixing circuit 221 is formed of mixer 223a, mixer 223b and phase shifter 225. Each of mixers 223a and 223b is supplied at its one input with the output of amplifier 219, while at its another input with the oscillation signal of local oscillator 227 via phase shifter 225. Mixing circuit 221 mixes amplifier 219's output signal and local oscillator 227's oscillation signal together, and converts the RF signal direct into I and Q signals whose phases are different to each other by 90 degrees. Local oscillator 227 is connected with PLL circuit 229 to form a loop.
Each of the I and Q signals is delivered to output terminal 233a, output terminal 233b, respectively, via low pass filter 231a, low pass filter 231b. The I and Q signals outputted from electronic tuner 205 are delivered to demodulation unit 207. Demodulation unit 207 is formed of A/D converters 235a and 235b supplied with the I and Q signals, respectively, and demodulator 237.
Since it is easy to integrate the above circuit of direct conversion system into an IC device, the direct conversion system circuit is suitable for use in portable television receiver 201 or the like apparatus where the easy-to-carry is the essential property. However, the direct conversion system has a disadvantage that the quality of receiving is deteriorated by a distortion of receiving signal due to DC offset voltage caused at mixing circuit 221.
As a countermeasure, the direct conversion system receiving circuit is provided with a control loop for suppressing the DC offset voltage. In the control loop, outputs of A/D converters 235a and 235b are delivered to detectors 239a and 239b, respectively. Detectors 239a and 239b detect DC offset voltage. Outputs of detectors 239a and 239b are supplied to control sections 241a and 241b, respectively, and the control sections generate cancellation signal for canceling the detected DC offset voltage.
Composers 243a and 243b are provided, respectively, between mixers 223a, 223b and low pass filters 231a, 231b. Composers 243a and 243b are supplied with the cancellation signal, and the DC offset voltage is cancelled.
Low power consumption is an essential requirement for RF receiving apparatus 201 used in a battery-operated portable television receiver or the like equipment. In the conventional RF receiving apparatus 201, however, control sections 241a and 241b always watch the signals outputted from detectors 239a and 239b, and generate the cancellation signal to be supplied to composers 243a and 243b. Thus, the control loop is always in the operating state and consumes a substantial power for the purpose.